I have a new 40 gal., nat. gas AO Smith water heater. The pilot light would not stay lit so I replaced the thermocouple. Lit the first time. Goes out after 1 tank heated up. I could not relight until the next day. Once again, relit for 1 tankful only. Now will not light again. Position in pilot flame looks good. No blockage in pilot line. Pilot flame blue and steady. What next? Thanks

That brand is trouble. If you bought a thermocouple at blue or orange, it may not be correct for that model. The other issue is supply of combustion air...is the WH located in a closet or a small room??

It could be the FVIR system, but they usally have to be reset if it is activated. Otherwise, there is no reason why it could not be relit immediately if it were a thermocouple problem, other than if it were getting too hot and had to cool down longer before it would work again. How new is it? If under warranty and you cannot diagnose the problem call AO Smith and have a factory warranty company check it out.

Air supply is good. moved the end of the TC around in case it was not directly in the flame. No help. Hard to see while in opperation. Still under warranty. I'll call AO for warrenty if the 2nd new TC does not help. (The wife is about to blow a gasket)